1. Field of the Invention
This invention is directed to apparatus for electrolytic printing. It is particularly concerned with the provision of a heated pressure pad in such apparatus which will significantly enhance printing quality and speed while reducing energy requirements.
2. Description of the Prior Art
The concept of electrically generated printing has sparked interest since the 1840's. Most attempts at utilizing an electrically initiated reaction by which printing could be accomplished required relatively high voltages, in the order of 100 to 250 volts as that term is used herein, saturated or completely wetted paper and/or consumable electrodes. It was also necessary to employ a recording medium which would be suitable for the particular printing system being used. Almost all of these prior art systems relied on either relatively high voltage pulses to achieve "dry" printing or on saturation of the recording medium to accomplish "wet" printing. As might be expected, there were also hybrid systems and recording mediums therefor that attempted to reconcile and/or compensate for the disadvantages of both the dry and wet approaches. However, as is the case with most compromise situations, these efforts were either too expensive to implement or unsatisfactory in output performance.
Various efforts were made to improve different aspects of the prior art printing systems. Among these were attempts directed to improving operating efficiency and/or print quality by pre-heating the print electrode, the recording medium or both. One such effort, which is related in subject matter to the present invention, was directed to heating a print stylus in order to improve the record formed thereby and is described in U.S. Pat. No. 2,454,966 to Faus. In this arrangement, a stylus formed from a resistance heating element, such as nichrome, is connected to a source of energy which heats the stylus when current flows therethrough. The heated stylus, in turn, rests on the lacquer coated surface of the recording medium employed and thus warms that surface prior to printing. The heated stylus renders the lacquer more transparent and softer, as well as more easily scraped off, to thereby expose a darker layer below the medium's surface.
U.S. Pat. No. 4,039,065 to Seki et al also discloses another effort to improve prior art printing apparatus by incorporating a preheating roller therein. The roller serves to heat the recording medium prior to printing and thereby lowers the total heat or energy that is needed by the print electrodes for application to the recording medium. The recording medium is thereby rendered more amenable to printing at a lower print electrode power level. However, while there was a reduction in the energy required at the print electrodes, there was no appreciable reduction in the total energy required to effect printing. This approach did lengthen print head life in the Seki et al apparatus.
The results of preheating were advantageous in printing systems based on electrolytic action and particularly useful in such a system where low voltage levels were employed to cause printing. One printing system that functions at low energy printing levels, of the magnitude associated with today's densely populated integrated circuit chips, is described in the commonly assigned U.S. patent application Ser. No. 237,560 filed on Feb. 24, 1981 by Bernier et al. In this arrangement, a leuco dye resident in the surface layer of the recording medium used therein is rendered visible by the application of a low energy pulse thereto providing the surface layer thickness, the contact surface area of the electrodes and the spacing between the electrodes are all set to predetermined values.
While operation of this printing arrangement would benefit from using one type or another of preheating device, those proposed in the prior art are not entirely suitable or satisfactory. The use of a preheated stylus or print electrodes, for example, would not be satisfactory in such a system for several reasons. The additional energy required at the stylus would not be compatible with the system's energy and voltage level constraints. In addition, a heated print stylus, by itself, would also detract from printing performance as it would further dry out the recording medium and retard the electrolytic reactions required to effect printing. The employment of a heated roller would also remove moisture from the recording medium and adversely affect print quality. Its effectiveness in increasing print head life would be more than offset by its additional energy use and dryness promotion. Furthermore, in neither prior art situation is the heating range of the stylus or roller limited or compensated for. Clearly, if the level of preheating cannot be assured or controlled, adverse printing results will occur before preventative or compensatory measures can be taken.
It has been found, as with most paper, that the recording medium tends to dry out between the time it is fabricated and the time it is actually used to print on. This situation, however, can be overcome in this particular printing environment by wetting the recording medium surface slightly, immediately prior to printing. Obviously, any effort to enhance print quality solely by heating the print stylus, using a preheated roller or some combination thereof in this particular low energy printing apparatus without compensating for the dryness problem would not be effective. The same would be true, in fact, for any electrolytic printing process whether it was low or high energy input driven.